The 'stacking rod' refers to the method of stacking three rifles vertically with their barrels crossing in a tripod shape in the field.

Keeps the actions out of the soil, snow and such. Mausers, Arisakas, and others used the cleaning rods under the barrel to do this.
In fact, many late war Arisakas only use a 'cleaning rod' that is a little over 4" to still allow the rifles to be stacked.

The U.S. Garands and '03s instead used the forward swivel that is notched to allow stacking since there was no cleaning rod.